Introduction to Python

Transcription

1 May 25, 2010

2 Basic Operators Logicals Types Tuples, Lists, & Dictionaries and or Building Functions

3 Labs From a non-lab computer visit: Running your own python setup, as well, is encouraged.

4 Book The book can be bought and shipped quickly for $40-$50. 2 copies will be on hold at Carpenter Library (1 in so far) It is on the order of recommended as in - required - but I can t get it into the bookshop quickly enough. You will use this book as long as you code in Python. We will cover highlights of Ch. 4 today. By the end of the semester you should know material from Ch.4, 5, 6, 7, 8, 9 off the top of your head, and have coding experience with Ch. 12, 13, 14, 15

5 Basic Operators Logicals Types Tuples, Lists, & Dictionaries Python has an interactive environment that allows you to try out assignments. It is very handy for learning and experimenting with everything we cover today. This environment is started by typing python (ipython if you have it) on unix systems(or Cygwin (or mac)). Python in windows varies on your installation.

6 Basic Operators Logicals Types Tuples, Lists, & Dictionaries +, -, *, / ordinary math operators a**b exponentiation, a b // division with rounding towards <<, >>, &, ˆ, bitwise operators (shifts, and, xor, or) <, <=, >, >=,!=, == comparisons

7 Basic Operators Logicals Types Tuples, Lists, & Dictionaries or, and, not in, not in is, is not boolean operators membership test identity test (point to the same thing)

8 Basic Operators Logicals Types Tuples, Lists, & Dictionaries hello, Hello strings 3, 4, 5 ints 3.14 floating points 3j imaginary numbers None None type

9 Basic Operators Logicals Types Tuples, Lists, & Dictionaries Tuples: 1. Immutable 2. Indexed from 0 3. Created with (content1, content2,...) or tuple(iterable), 4. Content can be any data type in Python, including tuples and mutable data.

10 Basic Operators Logicals Types Tuples, Lists, & Dictionaries Lists 1. Mutable 2. Indexed from 0 3. Very manipulable, with fast mutation implementation in Python 4. Will not use for now, but several built in functions.

11 Basic Operators Logicals Types Tuples, Lists, & Dictionaries Dictionaries 1. created with { key: value, key: value,...} or dict(key=values,...) 2. augmented with trial dict[key] = value 3. dictionaries are not ordered

12 Basic Functions Outline and or Building Functions Functional Programming is all about evaluating expressions. So, let s rewrite the function length (works only on lists): def Length(structure): if structure == []: return 0 else: return 1 + Length(structure[1:])

13 Let s look at some expressions: int(true) -> 1 int(false) -> 0 bool(7) -> True bool(true) -> True (0 == False) -> True (True and 7) -> 7 (7 and True) -> True (0 and False) -> 0 (False and 0) -> False and or Building Functions What s going on? Python moves left to right, evaluating each term as a boolean and returns the first value that allows determination of the expression as False or True.

14 and or Building Functions Python moves left to right, evaluating each term as a boolean and returns the first value that allows determination of the expression as False or True. (2 and 6 and 7) ->? (True and max) ->? not ( min and max) ->? (0 and False) ->?

15 and or Building Functions That was how Python evaluates the and in an expression. Here s how Python evaluates or : (True or 7) -> True (7 or True) -> 7 (0 or False) -> False (False or 0) -> 0 Python moves left to right, evaluating each term as a boolean and returns the first value that allows determination of the expression as False or True.

16 and or Building Functions Can we build things up with this? Normally: if cond1: fun1(x) else: fun2(x) Equivalent: (cond1 and fun1(x)) or (fun2(x))

17 and or Building Functions Back to rewriting the function Length with expressions only. def Length(structure): return (structure == [] and 0) or \ (1 + Length(structure[1:])) Problem: bool(([ ]==[ ] and 0)) = False never stops.

18 and or Building Functions Ex 1: def Length(structure): return int(not(structure==[]) and \ (1+FunctionalLength(structure[1:]))) Ex 2: def Length(structure): return (int(structure == []) or \ (2 + FunctionalLength(structure[1:]))) -1

19 Read: All submitted code will follow these guidelines.

20 tonight will be to read the style guide. You will be given a module who s code does not obey the style guide. You will be given an outline for a few simple recursive functions, fill them in.